The present invention relates to an overload protection for series resistances of electric motors, especially of fans of vehicles driven by electric motors.
Fans driven by electric motors are used to an ever-increasing extent for the forced ventilation of internal combustion engines, respectively radiators and passenger cells of vehicles, last by not least because they can be controlled relatively easily and require little driving energy. Thus, the heat balance of internal combustion engines can be regulated in an intended manner and the temperature, respectively, the rate of air flow in the passenger cell can be matched correctly to requirements.
However, frequently the wish exists, especially for reasons of the noise emission, to reduce the rotational speed of the fans in periods of reduced air requirement. As a rule, series or dropping resistances are utilized for that purpose, by means of which a single step or multi-step speed reduction can be achieved or a continuously variable speed reduction can be achieved by means of continuously variable series resistances.
Conditioned on principles, the series resistances are constructed of low-ohmic value and must be able to withstand a considerable thermal load. In case the fan is blocked, for example, by the entry of foreign bodies into its operating area or by a short-circuit at or in the electric motor, this may lead to an excessive heat-up and therewith eventually to a destruction of the series resistance without the occurrence of a response of the fuse in the current supply to the fan because the flowing excess-current is limited in part by the series resistance itself.
As solution to this problem, customarily the series resistances are either over-dimensioned or they are monitored by means of a thermal switch and are disconnected from the current supply.
Considerable additional costs as well as accommodation and heat removal problems are disadvantageous with the first solution. The second solution is also cost-intensive; additionally disadvantageous with the second solution is an adequate heat transfer from the resistance to the thermal switch which can be achieved only with great difficulty because both parts must be electrically insulated from one another at least within certain areas, which again implies a poor heat transfer.
A protective arrangement for starting resistances of motors against overload is discussed in the German Pat. No. 558,872. In this prior art circuit, a relay-controlled bridging of the resistance is also provided. However, the bridging does not serve the protection of the resistance during the blocking of the motor but rather for taking the resistance limiting the starting current out of the current circuit of the motor after starting has taken place. The starting resistance is thus not bridged or by-passed when the motor stands still (blocked) but, in contrast thereto, is additionally connected into the circuit. However, as its function is completely different from that of a rotational speed-reducing series resistance, it must also be dimensioned completely differently, to mention yet another difference. For the protection of this starting resistance, a slow-blowing fuse is provided within the bridging or by-pass circuit, directly in series with the same. However, disadvantageous with this circuit is the fact that an additional fuse is necessary as also the fact that its place of installation is determined by the location of the starting resistance, respectively, of its bridging or short-circuiting circuit. Furthermore, it is of disadvantage that the circuit is no longer functional once the fuse has responded because more simple blocking occurrences which could be eliminated by a higher starting torque, cannot be released therewith (the starting circuit is always set into operation when accelerating from the standstill).
A protective circuit for a d.c. motor is disclosed in the DE-OS No. 27 36 724. This circuit provides a series resistance connected in series with the motor. The voltage drop, which occurs across this series resistance and is proportional to the motor current, is monitored. If this voltage drop exceeds a limit value, then a disconnect signal is fed by way of a time-delay element to a switching contact connected in series with the motor.
The series resistance mentioned in this prior publication thus serves pure measuring purposes and is not suited for the current limitation for purposes of rotational speed reduction. An electronic circuit connected with the same thus monitors the motor and not a series resistance. For that reason, it must also be dimensioned completely differently; furthermore, by reason of the peculiarities of the circuit, it must be constructed safe against short-circuiting. Thus, a further series resistance would have to be used for the rotational speed reduction which under circumstances would have to be monitored with a separate device.
It is therefore the object of the present invention to provide an overload protection for series resistances of electric motors, especially for fans of vehicles driven by electric motors with one or several series resistances for the rotational speed reduction, which avoids the aforementioned disadvantages of electro-thermal overload protections, recognizes an increased load at least in one of these series resistances and safely protects the same against overload or destruction.
The underlying problems are solved according to the present invention in that during the occurrence of an increased power dissipation in at least one of these series resistances, a rotational speed step with full power output is selectively rendered operable.
The advantages of the present invention reside primarily in that an overload protection for series resistances of electric motors, especially of fans of a vehicle driven by electric motors is created which recognizes an occurring overload at series resistances that may be caused, for example, by a blocking of the fan, and reliably protects the series resistances. This is achieved with simple and cost-favorable means; the components necessary therefor require little space, can be accommodated without problems in a fan control apparatus and require no additional wiring expenditure.
These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, two embodiments in accordance with the present invention, and wherein: